I have the bard for the horn of berzerkers and the viola of shadow monsters (summon shadow 2 or 3, i think) and it is very, very helpfull. For summoning, there is an annoying limit of 6. I think the 3rd level shades are very worthwhile as well as the fire elementals. Mostly, good for blocking. The monster summon 6 is pretty cool too. Though i was a bit disappointed when it only summoned 2 wights; but those wights paralized tough lizardmen.
With 4 people in the party, and spells like mold touch and horrid wilting; lots of summoned creatures and webs are a necessity.